Diaphragms are used in various locations in the photographic art, such as in photographic enlargers, as well as in cameras. In the familiar single lens reflex camera, often referred to as an SLR camera, the diaphragm is preferably of the spring back type, opened wide to maximum aperture to admit maximum light during the focusing operation, and then closing down by spring action to a smaller aperture which has either been preset by the user of the camera or been automatically determined by the camera mechanism in response to a light measurement, this stopping down of the diaphragm occurring just before the exposure is to be made.
The space available in the housing or casing for the diaphragm is limited. According to the present invention, the external dimensions of the housing can be reduced in a radial direction (radial with respect to the optical axis) because two sets or series of diaphragm leaves are used instead of one, thereby enabling the leaves to be narrower in a radial direction with respect to the optical axis, so that they do not require so much radial space in the housing, when the leaves are in the fully open position.
An object of the present invention is to provide a spring back diaphragm for use especially but not exclusively in photographic cameras, and particularly in SLR cameras, in which the diaphragm, when fully open, has a larger free passage (that is, larger dimensional aperture) in proportion to the area of the annular space of the housing or mount which contains the diaphragm leaves when in their fully open position, than diaphragms of the prior art, and has a more favorable ratio of the area of the annular space of the housing to the size of the maximum aperture, than in diaphragms of the prior art.
For example, in one known diaphragm, when fully open to maximum aperture, there is a free passage having a diameter of 24 millimeters, and the diaphragm leaves lie in an annular space having an outside diameter of 45.2 mm when the diaphragm is fully open. In this known diaphragm, the area of the annular space required for housing the leaves in open position is more than 2.5 times the area of the maximum aperture, or a ratio of housing area to aperture area of approximately 2.5 to 1. As contrasted with this, a diaphragm according to the present invention can be made with a ratio of only about 1.5 to 1, as shown below.
This purpose is attained, in accordance with the invention, by providing the diaphragm leaves or blades in two groups or series, arranged in two different planes slightly spaced from each other in the direction of the optical axis. Each group or set of leaves arranged in the respective planes can be assigned a specific path upon the closing motion of the diaphragm, or the same path may be provided for both groups of leaves. The leaves can be mounted in such manner that upon closing the diaphragm, the leaves of both groups move in the same direction of rotation, or in opposite directions of rotation, as desired.
One advantageous embodiment of the invention comprises ten diaphragm leaves provided for the diaphragm, five of which are in one group, arranged in one plane, and the other five of which are in a second group, arranged in another plane spaced very slightly in an axial direction (axially of the optical axis) from those of the first group. The two groups of leaves move together, increasingly overlapping each other, until a predetermined diaphragm aperture is reached, whereupon the leaves of one group remain substantially stationary and only the leaves of the other group continue swinging further in a closing direction to close the diaphragm down to a still smaller aperture. The point at which the inward swinging movment of one group ceases can be chosen as desired, but is preferably the point at which an average or commonly used size of aperture is produced.
An advantage of the invention is the saving in outside dimensions of the mount or housing which holds the diaphragm leaves when they are retracted to maximum aperture position, as already mentioned above. By using ten leaves instead of the customary five leaves, each leaf can be made relatively narrow (in a direction radially with respect to the optical axis) and the first set of leaves, stopping their inward swinging movement before the minimum size aperture is reached, will serve to block light around the periphery of the light opening even when the leaves of the second group continue to move further inward to form the smallest aperture, which might bring the outer edges of these leaves inwardly beyond the periphery of the opening and might leave gaps at the periphery which, however, are effectively obstructed by the leaves of the other set. With this arrangement, a diaphragm can be constructed with, for example, a free passage through the fully opened diaphragm having a diameter of 34.5 mm., with an outside diameter of the annular space of the housing or mount being only 55.0 mm. The ratio of annular housing area to maximum aperture is only 1.5 to 1, which is quite favorable as compared with the previously mentioned prior art ratio of 2.5 to 1. Moreover, if it were attempted to make a diaphragm with a maximum aperture diameter of 34.5 mm. with only five leaves arranged in the conventional manner, the leaves would have to be much wider in a direction radially with respect to the optical axis, in order to avoid gaps or open spaces at the outer edge of the light opening when the diaphragm is stopped down to a small aperture, and this would result in the annular space for receiving the diaphragm leaves (when fully open) having an outside diameter of 65 mm, as compared with the outside diameter of only 55 mm with the present construction.